1. Field of the Invention
This invention relates to a phase change optical recording medium and a method for preparing such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a fluctuation of reflectance by the crystallographic change is detected for reproduction of the information. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used chalcogenite systems such as Ge-Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
It was also recently proposed to use new compounds known as chalcopyrites.
Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) Nos. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms when a recording layer crystallizes.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of high power (recording power) that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to gradually cool to recover the crystalline state.
In the optical recording medium of phase change type, dielectric layers are generally formed on opposite sides of the recording layer. Requirements for the dielectric layers are:
(1) the dielectric layers should be capable of protecting the recording layer and the substrate from heat histerisis by laser beam irradiation;
(2) the dielectric layers should be capable of amplifying the reproduced signal by making use of optical interference effect of lights reflected from boundaries between the layers; and
(3) the recording and erasing properties can be regulated by adjusting thermal conductivity and the like of each dielectric layer.
Typical dielectric layers which meet such requirements are those containing highly refractive ZnS as their main component. For example, Japanese Patent Application Kokai (JP-A) No. 103453/1988 discloses an optical information recording member having a dielectric layer containing a mixture of ZnS and SiO.sub.2. The merits described therein include increase in sensitivity for the power of incident light upon recording, and increase in the number of erasing/overwriting operations of the dielectric material. The increase in the sensitivity is said to have been realized by optimizing thermal constant of the dielectric layer, and the increase in the number of erasing/overwriting operations is said to have been realized by preventing the alteration in the nature of the dielectric layer.
However, it has been found in the investigation of the inventors of the present invention that use of dielectric layers containing zinc sulfide for the dielectric layer of an optical recording medium of phase change type results in marked increase of jitter and drop of C/N upon repeated overwriting, and hence, in reduced number of overwritable operations. A TDS analysis of ZnS-SiO.sub.2 dielectric layer revealed that the sulfur that had been incorporated in the dielectric layer as simple substance or dimer sulfur during the formation of the dielectric layer is released and sublimed when the dielectric layer is heated to about 400.degree. C., which is approximately the melting point of the recording layer, and consequently, the sulfur from the dielectric layer diffuses into the recording layer to invite change in the composition of the recording layer. It is assumed that the increase in jitter and the drop in C/N upon repeated overwriting is caused by such mechanism which in turn invites decrease in erasing rate and unstabilization of the distance between the record marks.